I'm not a blind user myself but I've worked with VoiceOver so hopefully some of this will help. I'm making this a community answer so feel free to add tips from your own experience if you've worked with UI layout in Xcode with VoiceOver.
When editing UI in Xcode there is an Inspector where you can change the size and position of the views. The shortcut to get to the Size Inspector is Alt+Command+5. The same shortcut works with VoiceOver.
You said that you were coding for both iOS and OS X. On iOS the y-axis starts in the top and points downward (so a higher y value means that the view is lower on the screen). This means that x and y specify the upper left corner of the view and the width and height extends to the right and down from there.
On OS X it's the opposite. The y-axis start at the bottom and a higher y value means that the view is higher on the screen. In both cases the x-axis goes from left to right. This means that x and y specifies the lower left corner and the width and height extends to the right an up.
Further, each view is positioned relative to their parent instead of absolute coordinates. This means that if you position a view at x=10 and y=30 and then position another view inside that at x=5 and y=10 it will have an absolute position on the screen that is x=15 and y=40.
If you can picture the layout in your head then you should be able to do it like this but it may still be hard to do.
Update
At the top of the hierarchy these coordinates relate to the size of the window. On iOS you have fixed sizes (320×480 for the iPhone and 1024×768 for the iPad). Depending on if the device is in landscape or portrait one of these is the width and the other is the heigh. You usually subtract 20 pixels from the height to account for the status bar. So the coordinate where y=0 would be directly below the status bar.
On OS X you can change the size of the window yourself. I will try and explain where to find it.
At the top level. Navigate to the "source code group" and interact with it. There you should find a "navigation bar group" and a "table" and a "scroll area". Interact with the table. In that table you should find a list of Placeholders and a list of Objects. One of the Objects will be the "Window". With the Window selected, all the inspectors will change properties of the Window. You can quickly jump to the Size Inspector by changing to any inspector and then back to the Size Inspector. For exaple type Alt+Command+4 and then Alt+Command+5. The first two elements in the Size Inspector should be the Width and Height of the Window.